The printhead of an impact-type dot matrix printer includes a plurality of individual driver mechanisms, usually one driver for each dot position in one or more columns of the matrix. The distance through which the driver moves an impact element to print a dot on a record-receiving sheet is quite small (e.g., of the order of 0.02 inch), allowing for high speed operation. in a column-sequential printer having a nominal print rate of thirty characters per second, and using a 5.times.7 dot matrix for each character with three blank columns between characters, for example, the maximum print rate may actually be fifty to sixty characters per second or 400 to 480 columns/second. The driver mechanism is usually an electromagnetic device of either the solenoid or clapper type, having a small air gap between two magnetizable surfaces which attract each other when the coil of the driver is energized. Electromagnetic drivers of this kind are well suited to produce the required impact force while operating over a very small stroke. An effective direct-acting solenoid driver of this general type is shown in Zenner et al U.S. Pat. No. 3,729,079.
A conventional clapper or solenoid type electromagnetic driver, however, has one important disadvantage as applied to a dot matrix printhead. The force developed by the electromagnetic driver is small at the beginning of the stroke and becomes much larger at the end of the stroke. This is precisely the opposite of the optimum force/travel characteristic. A small force at the beginning of the stroke wastes time in getting the impact element to move, and there is little or no advantage to application of a large force later when the print rod or other impact element is about to hit the paper. Ideally, a printhead driver for an impact-type dot matrix printer should develop a large force at the beginning of its stroke, for maximum acceleration, and that force should diminish as the impact element approaches the paper or other record-receiving sheet. The impact on the paper then becomes largely independent of the length of travel, allowing for a substantial tolerance for the spacing between the platen of the printer and the impact elements of the printhead.
One basic mechanism that affords a close approximation to the ideal force/travel characteristic referred to above is a spring. An impact element driven by a spring provides maximum force at the beginning of its stroke; furthermore, that force decreases approximately linearly as the print element moves toward the platen. For optimum operation, the force should become approximately zero at the point where the print rod or other impact element would strike multi-copy paper, with a reverse force coming into effect for further travel of the impact element. This affords an automatic force adjustment for the number of copies being produced, with maximum impact on relatively thick multi-copy paper but with reduced impact when the print element travels further to strike a single sheet of paper.
A force/travel characteristic of this general type is provided in some known printheads, including for example the commercially available Teletype 43 printer, and the printhead drivers described in Baumeister et al. U.S. Pat. No. 4,000,801 and Ek et al. U.S. Pat. No. 4,109,776. In the Ek and Baumeister mechanisms, the impact element for each driver is mounted on a spring that also carries a magnetic armature. The spring is normally held in a cocked position by an electromagnet that is held energized and that attracts the armature on the spring. To print a dot, the electromagnet is de-energized, releasing the spring to move toward an unflexed position, this movement constituting the print impact movement for the print rod or other element mounted on the spring. But a printhead drive of this kind is inherently inefficient as compared with one in which an electromagnet is used in a direct drive relationship to the dot impact element, because the electromagnet coil is maintained energized most of the time instead of being energized only momentarily for each print stroke. The Teletype 43 mechanism is similar but uses a permanent magnet to hold the spring in cocked position; to print a dot, an electromagnet is energized to overcome the permanent magnet flux, releasing the spring for printing movement. This results in improved energy efficiency, but the printhead is rather heavy and bulky due to the permanent magnet structure.